Integrated circuit chip packages are commonly connected to printed circuit boards to achieve an overall electronic function. The connection between the integrated circuit chip package and the printed circuit board typically allows for the conduction of electricity from the circuitry of the integrated circuit chip to circuitry or other components on the printed circuit board. One method of making the connection between the integrated circuit chip package and the printed circuit board is to use one or more ball connectors, such as solder balls, in what is known as a flip-chip or an area array packaging.
One problem with solder ball connections arises from the difference between the coefficient of thermal expansion of the integrated circuit chip package and that of the printed circuit board. The coefficients of thermal expansion of certain integrated circuit chip packages are largely governed by the expansion characteristics of the silicon which may have, for example, a coefficient of thermal expansion on the order of about three ppm/.degree. C. The coefficient of thermal expansion of the printed circuit board is largely a function the amount of copper incorporated into the printed circuit board. The circuit board may have, for example, a coefficient of thermal expansion on the order of about seventeen ppm/.degree. C. Therefore, the difference in thermal expansion between the integrated circuit chip package and the printed circuit board of this example is about fourteen ppm/.degree. C. Typically, the difference in thermal expansion is in the range of about ten to fifteen ppm/.degree. C.
This difference in thermal expansion results in relatively large stresses on the solder ball connectors which form the connection between the integrated circuit chip package and the printed circuit board. Over the life of a typical printed circuit board and integrated circuit chip package combination, cycles of thermal expansion and contraction result in damage to the solder ball connections. Thermal expansion or contraction can be caused by any number of factors including heat generated by the flow of electricity within the chip of the integrated circuit chip package or on the printed circuit board, environmental factors such as normal heat and cooling, or exposure of a printed circuit board arrangement to heating and cooling cycles upon power-on and power-off cycles during operation of the particular electronic device incorporating the printed circuit board.
In one type of a flip-chip arrangement, a first set of solder ball connectors is provided on the integrated circuit chip surface. This first set of ball connectors is used to connect the integrated circuit chip to an interposer which is an intermediate layer between the integrated circuit chip and the printed circuit board. The first set of ball connectors may constitute, for example, C4-type solder balls. The interposer is connected to the printed circuit board by a second set of ball connectors. These may constitute, for example, C5-type solder balls. In one type of an area array package, the integrated circuit chip is wire bonded to the interposer. A set of ball connectors is used to connect the interposer to the printed circuit board. These ball connections suffer stresses due to the thermal expansion characteristics discussed above. This can cause damage or fatigue to the ball connections.
One method of relieving stresses within the ball connections is to provide a buffer material between the system level printed circuit board and the integrated circuit chip. For example, a low modulus buffer layer, such as silicone, may be provided between the interposer and the system level printed circuit board. Stresses caused by differences in thermal expansion are partially absorbed by the silicone. However, this gives rise to high stresses in the solder balls which connect the interposer to the integrated circuit chip. Those having ordinary skill in the relevant art will recognize that other problems exist with prior solder ball connections between integrated circuit chips, interposers and printed circuit boards.